Title: Gold Nanoparticles on Ceria: Importance of O Vacancies in the Activation of Gold

Abstract

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO{sub 2} and the water-gas shift (WGS, CO + H{sub 2}O {yields} H{sub 2} + CO{sub 2}) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO{sub 2} was investigated on Au/CeO{sub 2}(111) and AuO{sub x} /CeO{sub 2} surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO{sub 2}(111) was 4-7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO{sub 2} on the Au/CeO{sub 2}(111) surfaces. The full decomposition of SO{sub 2} was observed only after introducing O vacancies in the ceria support. AuO{sub x} /CeO{sub 2} surfaces were found to be much less chemically active than Au/CeO{sub 2}(111) or Au/CeO{sub 2-x} (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured (Au + AuO{sub x})-CeO{sub 2} catalysts under the WGS reaction. At temperatures above 250 C, a complete AuO{sub x} {yields} Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria filmsmore » or a CeO{sub 2}(111) single crystal corroborate that cationic Au{sup {delta}+} species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in (Au + AuO{sub x})/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.« less

@article{osti_929928,
title = {Gold Nanoparticles on Ceria: Importance of O Vacancies in the Activation of Gold},
author = {Rodriguez,J. and Wang, X. and Liu, P. and Wen, W. and Hanson, J. and Hrbek, J. and Perez, M. and Evans, J.},
abstractNote = {Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO{sub 2} and the water-gas shift (WGS, CO + H{sub 2}O {yields} H{sub 2} + CO{sub 2}) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO{sub 2} was investigated on Au/CeO{sub 2}(111) and AuO{sub x} /CeO{sub 2} surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO{sub 2}(111) was 4-7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO{sub 2} on the Au/CeO{sub 2}(111) surfaces. The full decomposition of SO{sub 2} was observed only after introducing O vacancies in the ceria support. AuO{sub x} /CeO{sub 2} surfaces were found to be much less chemically active than Au/CeO{sub 2}(111) or Au/CeO{sub 2-x} (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured (Au + AuO{sub x})-CeO{sub 2} catalysts under the WGS reaction. At temperatures above 250 C, a complete AuO{sub x} {yields} Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO{sub 2}(111) single crystal corroborate that cationic Au{sup {delta}+} species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in (Au + AuO{sub x})/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.},
doi = {10.1007/s11244-007-0280-1},
journal = {Topics in Catalysis},
number = 39449,
volume = 44,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Ab initio Molecular Dynamics simulations and static Density Functional Theory calculations have been performed to investigate the reaction mechanism of CO oxidation on Au/CeO 2 catalyst. It is found that under reaction condition CO adsorption significantly labializes the surface atoms of the Au cluster and leads to the formation of isolated Au+-CO species that resides on the support in the vicinity of the Au particle. In this context, we identified a dynamic single-atom catalytic mechanism at the interfacial area for CO oxidation on Au/CeO 2 catalyst, which is a lower energy pathway than that of CO oxidation at the interfacemore » with the metal particle. This results from the ability of the single atom site to strongly couple with the redox properties of the support in a synergistic manner thereby lowering the barrier for redox reactions. We find that the single Au+ ion, which only exists under reaction conditions, breaks away from the Au cluster to catalyze CO oxidation and returns to the Au cluster after the catalytic cycle is completed. Generally, our study highlights the importance of the dynamic creation of active sites under reaction conditions and their essential role in a catalytic process.« less

We used scanning tunnelling microscopy to study the morphology of an overlayer of ceria in contact with a TiO 2(110) substrate. Two types of domains were observed after ceria deposition. An ordered ceria film covered half of the surface and high-resolution imaging suggested a near-c(6 × 2) relationship to the underlying TiO 2(110)-(1 × 1). For the other half of the surface, it comprised CeO x nanoparticles and reconstructed TiOx supported on TiO 2(110)-(1 × 1). Exposure to a small amount of gold resulted in the formation of isolated gold atoms and small clusters on the ordered ceria film andmore » TiO 2(110)-(1 × 1) areas, which exhibited significant sintering at 500 K and showed strong interaction between the sintered gold clusters and the domain boundaries of the ceria film. The Au/CeO x/TiO 2(110) model system proved to be a good catalyst for the water–gas shift (WGS) exhibiting much higher turnover frequencies (TOFs) than Cu(111) and Pt(111) benchmarks, or the individual Au/TiO 2(110) and Au/CeO 2(111) systems. Finally, for Au/CeO x/TiO 2(110) catalysts, there was a decrease in catalytic activity with increasing ceria coverage that correlates with a reduction in the concentration of Ce 3 + formed during WGS reaction conditions.« less

In this study, scanning tunnelling microscopy has been used to study the morphology of an overlayer of ceria in contact with a TiO 2(110) substrate. Two types of domains were observed after ceria deposition. An ordered ceria film covered half of the surface and high-resolution imaging suggested a near-c(6 × 2) relationship to the underlying TiO 2(110)-(1 × 1). The other half of the surface comprised CeO x nanoparticles and reconstructed TiOx supported on TiO 2(110)-(1 × 1). Exposure to a small amount of gold resulted in the formation of isolated gold atoms and small clusters on the ordered ceriamore » film and TiO 2(110)-(1 × 1) areas, which exhibited significant sintering at 500 K and showed strong interaction between the sintered gold clusters and the domain boundaries of the ceria film. The Au/CeO x/TiO 2(110) model system proved to be a good catalyst for the water–gas shift (WGS) exhibiting much higher turnover frequencies (TOFs) than Cu(111) and Pt(111) benchmarks, or the individual Au/TiO 2(110) and Au/CeO 2(111) systems. For Au/CeO x/TiO 2(110) catalysts, there was a decrease in catalytic activity with increasing ceria coverage that correlates with a reduction in the concentration of Ce 3+ formed during WGS reaction conditions.« less